变转速耦合偏心结构实现高效层流混合的荧光可视化试验研究

Mixing Enhancement by Variable Rotational Speed Coupling Eccentric Structure and Its Flow Field Visualization by PLIF

机械搅拌广泛应用于化工、食品、冶金、环保等过程工业领域。层流搅拌工况下,搅拌槽内由于搅拌桨叶的规律性扰动使桨叶的上下方产生环状隔离流场。隔离流场的封闭性使流场内外工质无法交换,成为实现高效层流混合的主要障碍。采用平面激光诱导荧光(Planner laser induced fluorescence,PLIF)技术显示瞬态流场结构。通过编写图像处理程序,识别非混合区域并定量计算出混合效率。试验结果表明,传统中心搅拌的混合效率停留在63%左右,经过多个周期的变转速搅拌后混合效率可提升至约78%。搅拌轴的偏心布置破坏了流场的周期对称性,促使隔离区域减小。当偏心率E=0.1、0.3、0.4、0.5、0.6时,在200 s内混合效率分别为79%、85%、89%、90%、86%。研究发现偏心率与混合效率并非单调线性关联,偏心率存在最优值范围(E=0.4~0.5)。然而流动隔离区仅通过搅拌轴的偏心布置仍然无法完全消除。提出变转速耦合搅拌轴偏心布置的策略,通过施加非定常非对称扰动实现层流搅拌工况下的高效混合。荧光可视化结果表明,与中心变转速搅拌相比,偏心变转速搅拌对流场的改变更为明显。当偏心率E=0.3、0.4、0.5时,经过三个周期的变转速层流混合,隔离区域大幅缩减,混合效率最高可达97%以上。将为高效层流搅拌混合器的设计提供技术与理论指导。

Mechanical mixing has been widely used in chemical engineering,food industry,metallurgy and environmental protections.In laminar mixing,doughnut-shaped toroidal regions,which are also termed as isolated mixing regions(IMRs),appear above and below the each impeller in the vessel due to the periodic flow structure.Fluids can neither penetrate nor leave the islands.Therefore,IMRs are barriers to mixing.In this work,it is employed a planar laser induced fluorescence(PLIF)technology to visualize the dynamic structure of the flow field.The post-image processing has been done with custom-made functions which could identify the IMRs and calculates the mixing efficiency.The results show that the mixing efficiency of traditional center mixing stays at about 63%.The mixing efficiency could be improved to 78%approximately by applying time-periodic RPM fluctuations.The eccentric arrangement of the stirring shaft destroys the periodic symmetry of the flow field and thus reduces the IMRs.When the eccentricity E=0.1,0.3,0.4,0.5,0.6,the mixing efficiency could achieve 79%,85%,89%,90%,86%in 200 s,respectively.The results demonstrate that eccentricity and mixing efficiency are not linearly related.There is an optimal range of eccentricity(E=0.4-0.5)for the best mixing efficiency.However,the IMRs cannot be completely eliminated only by arranging the stirring shaft eccentrically.Thus,it proposed a strategy to combine the eccentric arrangement of stirring shafts with variable rotational speeds,which might improve the mixing quality dramatically by imposing unsteady asymmetric disturbances.The PLIF images exhibit that the flow structures have been remarkable alternated by eccentric stirring with variable rotational speeds.Under the eccentricities E=0.3,0.4,0.5,the IMRs has been greatly reduced by variable rotational speeds subjected to time-periodic fluctuations.Specifically,the mixing efficiency can reach more than 97%.This research could provide technical and theoretical guidance for the design of laminar flow agitators.